1. Field of the Invention
This invention relates generally to a method for estimating vehicle side-slip and, more particularly, to a method for estimating vehicle side-slip within a linear vehicle operating region, where the method includes updating front and rear cornering stiffness signals using a linear bicycle model with state feedback.
2. Discussion of the Related Art
Various vehicle control systems are known in the art, such as lane-keeping systems, lane-changing systems and path-following systems, that improve driver convenience, safety and comfort. These control systems typically operate within a linear vehicle operating region, where the vehicle states define the behavior of the vehicle. Vehicle side-slip is one of the key states for determining vehicle dynamics, kinematics and control for these types of control systems. Vehicle side-slip is defined as the lateral speed at the vehicle's center of gravity in a direction perpendicular to the vehicle longitudinal velocity. The vehicle side-slip combined with the vehicle longitudinal velocity forms a vehicle vector velocity in the vehicle traveling direction. Vehicle control systems calculate the vehicle side-slip to determine an error so that the vehicle can be controlled to reduce the side-slip to zero. However, vehicle side-slip is difficult to accurately calculate because vehicle side-slip is typically very small within the linear operating region of the vehicle. Also, measuring the vehicle lateral speed to determine the side-slip is currently cost prohibitive, and thus impractical.
Three approaches are known in the art for determining vehicle side-slip. These approaches are sometimes defined as an indirect sensor approach, a direct sensor approach and a model-based approach. The indirect sensor approach estimates vehicle side-slip by the integration of the side-slip rate from existing sensors, such as yaw rate sensors and lateral acceleration sensors. However, this approach typically has the disadvantages of sensor noise, drift and cumulative integration error.
The direct sensor approach measures the vehicle side-slip directly, such as by a GPS or camera based system. However, this approach typically has a slow throughput, and is often subject to weather and/or road conditions, and thus may not be suitable for fast-loop control purposes and be reliable for all times.
The model based approach is valid only in the vehicle linear operating region, but has the advantages of being more reliable and cost effective. The model based side-slip estimation results are more smooth and continuous, and the requirements of the sensors are less restrictive. However, the model based approach is often limited by model inaccuracy, unknown road surfaces and other unknown disturbances.